Move from MatrixDotNet.NetCore to multitargeting other projects

[Dilorfin]

It seems to me, that here we have strange idea of adding one more project with the same functionality (but with other names) for .NetCore3. Let's move forward to removing this idea with multitargeting our code in MatrixDotNet and MatrixDotNet.Math project.

Code sample:

#if NETSTANDARD2_1
    Console.WriteLine("netstandard21");
#elif NETCOREAPP3_1
    Console.WriteLine("net31");
#endif

.csproj file:

<!-- this will transform -->
<TargetFramework>netstandard2.1</TargetFramework>
<!-- to this -->
<TargetFrameworks>netstandard2.1;netcoreapp3.1</TargetFrameworks>

Useful link: https://docs.microsoft.com/en-us/dotnet/standard/frameworks https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives/preprocessor-if